Managing Systems and Workgroups: A Guide for HP-UX System Administrators

Using High Availability Strategies
Using Software Mirroring as a Disk Protection Strategy
Appendix A948
Using Software Mirroring as a Disk
Protection Strategy
Data redundancy is necessary to prevent instances in which a single disk
failure can cause a system to go down until the problem is located and
corrected. There are two methods of providing data redundancy: software
mirroring and hardware mirroring. Each represents RAID Level 1. (See
“Using Disk Arrays” on page 949 for more information on the meaning of
the various RAID levels.)
Software mirroring allows you to maintain identical copies of your
data (except for the root disk), so that each set of data has, in effect, a
perfect clone of itself. In the event a disk fails, the system can use the
mirrored copy of the data, thus allowing users to continue to work
without interruption. The bad disk can be replaced at a more convenient
time when the system can be brought down without causing problems.
Once the system is rebooted, the mirroring software will cause the
mirrored data to be copied back to the replacement disk and the process
of mirroring will begin again.
With three-way disk mirroring, two copies of each disk’s data are
maintained. This strategy is even more robust than two-way mirroring
which is described above and it eliminates the need to bring the system
down at all in order to replace a bad disk.
To use these types of disk mirroring, you will need to use LVM or VxVM
as your disk management strategy and have available the
MirrorDisk/UX software product. MirrorDisk/UX causes every write to
the original disk to also be written to the copy or copies of the original
disk. Note that the original data and its copied data may be spread over
more than one disk.
The main advantage of software mirroring over hardware mirroring,
which is discussed in “Using Disk Arrays” on page 949, is that the cost of
implementation is lower. The main disadvantage of software mirroring
relates to its increased complexity of management. That is, it will
probably be significantly more difficult to manage a system with a large
number of disks as compared to a system with a single disk array.